Struggling artist Eben Adams (Joseph Cotten) finds a sympathetic patron in 1940s New York City art gallery owner, MIss Spinney (Ethel Barrymore). Afterwards, as he rests in Central Park, he meets a lovely, but mysterious, young girl, Jennie Appleton (Jennifer Jones). She sings him a haunting song about "where I come from, nobody knows, and where I'm going, everything goes." Jennie says that her parents were performing acrobats at Hammersteins theatre, which Eben knows was closed years ago. After she leaves, he finds a scarf she left behind, wrapped in a 30 year old newspaper.

Eben is moved to make sketches of Jennie as he remembers her, and Miss Spinney and her co-owner, Matthews (Cecil Kellaway) are impressed with the talent that has emerged from his inspiration.

A few weeks later he again encounters Jennie ice skating in Central Park, but she seems much older and taller than their first meeting. She is delighted when Eben proposes to paint a portrait of her. They agree to meet on the following Saturday.

When Jennie fails to show for their meeting, Eben decides to find out what he can about her parents. He visits an old woman from the theatre who has scrapbook photos from the early 1900s of the Appletons, including, to Eben's amazement, a picture of young Jennie. As he later walks in the park, he again finds Jennie, this time distraught over the death of her parents in a high wire accident. She says that her aunt is sending to a convent. She tells Eben, "You'll wait for me, won't you?"

As months go by, Eben is obsessed with her memory and believes he won't accomplish anything worthwhile until she returns for her portrait.

One night he returns to his apartment and Jennie is there waiting for him, again appearing as if years have passed. He paints the portrait he has long been waiting to do. Later he visits her at her convent, where she looks in his eyes and says, "I'm so glad you're waiting for me, Eben."

He shows the unfinished portrait to Miss Spinney and Matthews who pronounce it a masterpiece. Matthews effuses, "What you see in that face is without age or time."

When he again finds Jennie, they return to his apartment to finish the portrait. She tells him they will meet again at the end of the summer. He turns away for a moment, and she is gone.

When summer turned to fall and Jennie hasn't returned, he goes to the convent to ask the sisters about her. Mary Mother of Mercy (Lillian Gish) tells him that Jennie died, many years ago, when a tidal wave struck at Cape Cod where she was vacationing. Eben realizes that the anniversary of her death is only a few days away, and he hurries there, hoping to save her.

He rents a boat to sail to Lands End Point. As he approaches the lighthouse, a ferocious storm builds (the storm sequence is green-tinted in the otherwise black and white movie), he capsizes and is washed ashore on the island. Struggling to the top of the lighthouse, he looks out at the stormy sea and sees Jennie's sailboat washing on to the shore. He rushes to her and, as they embrace, a giant wave crashes over and separates them. Eben survives and is rescued by the locals, and Jennie's scarf is found nearby.

The final scene is in a large art gallery, where three young girls admire the final Portrait of Jennie (presented in full Technicolor).